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.TH RECVMSG 3XNET "September 10, 2018"
.SH NAME
recvmsg \- receive a message from a socket
.SH SYNOPSIS
.LP
.nf
\fBcc\fR [ \fIflag\fR ... ] \fIfile\fR ... \fB-lxnet\fR [ \fIlibrary\fR ... ]
#include <sys/socket.h>

\fBssize_t\fR \fBrecvmsg\fR(\fBint\fR \fIsocket\fR, \fBstruct msghdr *\fR\fImessage\fR, \fBint\fR \fIflags\fR);
.fi

.SH DESCRIPTION
.LP
The \fBrecvmsg()\fR function receives a message from a connection-mode or
connectionless-mode socket. It is normally used with connectionless-mode
sockets because it permits the application to retrieve the source address of
received data.
.sp
.LP
The \fBrecvmsg()\fR function receives messages from unconnected or connected
sockets and returns the length of the message.
.sp
.LP
The \fBrecvmsg()\fR function returns the total length of the message. For
message-based sockets such as \fBSOCK_DGRAM\fR and \fBSOCK_SEQPACKET\fR, the
entire message must be read in a single operation. If a message is too long to
fit in the supplied buffers, and \fBMSG_PEEK\fR is not set in the \fIflags\fR
argument, the excess bytes are discarded, and \fBMSG_TRUNC\fR is set in the
\fBmsg_flags\fR member of the \fBmsghdr\fR structure. For stream-based sockets
such as \fBSOCK_STREAM\fR, message boundaries are ignored. In this case, data
is returned to the user as soon as it becomes available, and no data is
discarded.
.sp
.LP
If the \fBMSG_WAITALL\fR flag is not set, data will be returned only up to the
end of the first message.
.sp
.LP
If no messages are available at the socket, and \fBO_NONBLOCK\fR is not set on
the socket's file descriptor, \fBrecvmsg()\fR blocks until a message arrives.
If no messages are available at the socket and \fBO_NONBLOCK\fR is set on the
socket's file descriptor, the \fBrecvmsg()\fR function fails and sets
\fBerrno\fR to \fBEAGAIN\fR or \fBEWOULDBLOCK\fR.
.sp
.LP
In the \fBmsghdr\fR structure, defined in \fBsocket.h\fR(3HEAD), the
\fBmsg_name\fR and \fBmsg_namelen\fR members specify the source address if the
socket is unconnected. If the socket is connected, the \fBmsg_name\fR and
\fBmsg_namelen\fR members are ignored. The \fBmsg_name\fR member may be a null
pointer if no names are desired or required.
.sp
.LP
The \fBmsg_control\fR and \fBmsg_controllen\fR members specify a buffer to
receive ancillary data sent along with a message. Ancillary data consists of a
sequence of pairs. Each pair is composed of a \fBcmsghdr\fR structure followed
by a data array. The \fBcmsghdr\fR structure, defined in \fBsocket.h\fR(3HEAD),
contains descriptive information which allows an application to correctly parse
data. The data array contains the ancillary data message.
.sp
.LP
If ancillary data is not transferred, \fBmsg_control\fR is set to \fBNULL\fR
and \fBmsg_controllen\fR is set to \fB0\fR.
.sp
.LP
The \fBmsg_iov\fR and \fBmsg_iovlen\fR fields of the \fBmsghdr\fR structure are
used to specify where the received data will be stored. \fBmsg_iov\fR points to
an array of \fBiovec\fR structures. The \fBmsg_iovlen\fR must be set to the
dimension of this array. In each \fBiovec\fR structure, the \fBiov_base\fR
field specifies a storage area and the \fBiov_len\fR field gives its size in
bytes. Each storage area indicated by \fBmsg_iov\fR is filled with received
data in turn until all of the received data is stored or all of the areas have
been filled.
.sp
.LP
If the \fBSO_TIMESTAMP\fR option has been enabled through \fBsetsockopt()\fR,
then a struct \fBtimeval\fR is returned following the \fBcmsghdr\fR, and the
\fBcmsg_len\fR field of the \fBcmsghdr\fR indicates the size of the struct
\fBtimeval\fR.
.sp
.LP
On successful completion, the \fBmsg_flags\fR member of the message header is
the bitwise-inclusive OR of all of the following flags that indicate conditions
detected for the received message:
.sp
.ne 2
.na
\fB\fBMSG_EOR\fR\fR
.ad
.RS 14n
End of record was received (if supported by the protocol).
.RE

.sp
.ne 2
.na
\fB\fBMSG_OOB\fR\fR
.ad
.RS 14n
Out-of-band data was received.
.RE

.sp
.ne 2
.na
\fB\fBMSG_TRUNC\fR\fR
.ad
.RS 14n
Normal data was truncated.
.RE

.sp
.ne 2
.na
\fB\fBMSG_CTRUNC\fR\fR
.ad
.RS 14n
Control data was truncated.
.RE

.SH PARAMETERS
.LP
The function takes the following arguments:
.sp
.ne 2
.na
\fB\fIsocket\fR\fR
.ad
.RS 11n
Specifies the socket file descriptor.
.RE

.sp
.ne 2
.na
\fB\fImessage\fR\fR
.ad
.RS 11n
Points to a \fBmsghdr\fR structure, containing both the buffer to store the
source address and the buffers for the incoming message. The length and format
of the address depend on the address family of the socket. The \fBmsg_flags\fR
member is ignored on input, but may contain meaningful values on output.
.RE

.sp
.ne 2
.na
\fB\fIflags\fR\fR
.ad
.RS 11n
Specifies the type of message reception. Values of this argument are formed by
logically OR'ing zero or more of the following values:
.sp
.ne 2
.na
\fB\fBMSG_OOB\fR\fR
.ad
.RS 15n
Requests out-of-band data. The significance and semantics of out-of-band data
are protocol-specific.
.RE

.sp
.ne 2
.na
\fB\fBMSG_PEEK\fR\fR
.ad
.RS 15n
Peeks at the incoming message.
.RE

.sp
.ne 2
.na
\fB\fBMSG_WAITALL\fR\fR
.ad
.RS 15n
Requests that the function block until the full amount of data requested can be
returned. The function may return a smaller amount of data if a signal is
caught, if the connection is terminated, if \fBMSG_PEEK\fR was specified, or if
an error is pending for the socket.
.RE

.RE

.SH USAGE
.LP
The \fBselect\fR(3C) and \fBpoll\fR(2) functions can be used to determine when
data is available to be received.
.SH RETURN VALUES
.LP
Upon successful completion, \fBrecvmsg()\fR returns the length of the message
in bytes. If no messages are available to be received and the peer has
performed an orderly shutdown, \fBrecvmsg()\fR returns \fB0\fR. Otherwise,
\fB\(mi1\fR is returned and \fBerrno\fR is set to indicate the error.
.SH ERRORS
.LP
In addition to the errors documented below, an asynchronous error generated by
the underlying socket protocol may be returned. For the full list of errors,
please see the corresponding socket protocol manual page. For example, for a
list of TCP errors, please see \fBtcp\fR(4P).
.sp
.LP
The \fBrecvmsg()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEAGAIN\fR\fR
.ad
.br
.na
\fB\fBEWOULDBLOCK\fR\fR
.ad
.RS 15n
The socket's file descriptor is marked \fBO_NONBLOCK\fR and no data is waiting
to be received; or \fBMSG_OOB\fR is set and no out-of-band data is available
and either the socket's file descriptor is marked \fBO_NONBLOCK\fR or the
socket does not support blocking to await out-of-band data.
.RE

.sp
.ne 2
.na
\fB\fBEBADF\fR\fR
.ad
.RS 15n
The \fIsocket\fR argument is not a valid open file descriptor.
.RE

.sp
.ne 2
.na
\fB\fBECONNRESET\fR\fR
.ad
.RS 15n
A connection was forcibly closed by a peer.
.RE

.sp
.ne 2
.na
\fB\fBEFAULT\fR\fR
.ad
.RS 15n
The \fImessage\fR parameter, or storage pointed to by the \fImsg_name\fR,
\fImsg_control\fR or \fImsg_iov\fR fields of the \fImessage\fR parameter, or
storage pointed to by the \fBiovec\fR structures pointed to by the
\fImsg_iov\fR field can not be accessed or written.
.RE

.sp
.ne 2
.na
\fB\fBEINTR\fR\fR
.ad
.RS 15n
This function was interrupted by a signal before any data was available.
.RE

.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 15n
The sum of the \fBiov_len\fR values overflows an \fBssize_t\fR. or the MSG_OOB
flag is set and no out-of-band data is available.
.RE

.sp
.ne 2
.na
\fB\fBEMSGSIZE\fR\fR
.ad
.RS 15n
The \fBmsg_iovlen\fR member of the \fBmsghdr\fR structure pointed to by
\fImessage\fR is less than or equal to 0, or is greater than \fBIOV_MAX\fR.
.RE

.sp
.ne 2
.na
\fB\fBENOTCONN\fR\fR
.ad
.RS 15n
A receive is attempted on a connection-mode socket that is not connected.
.RE

.sp
.ne 2
.na
\fB\fBENOTSOCK\fR\fR
.ad
.RS 15n
The \fIsocket\fR argument does not refer to a socket.
.RE

.sp
.ne 2
.na
\fB\fBEOPNOTSUPP\fR\fR
.ad
.RS 15n
The specified flags are not supported for this socket type.
.RE

.sp
.ne 2
.na
\fB\fBETIMEDOUT\fR\fR
.ad
.RS 15n
The connection timed out during connection establishment, or due to a
transmission timeout on active connection.
.RE

.sp
.LP
The \fBrecvmsg()\fR function may fail if:
.sp
.ne 2
.na
\fB\fBEIO\fR\fR
.ad
.RS 11n
An \fBIO\fR error occurred while reading from or writing to the file system.
.RE

.sp
.ne 2
.na
\fB\fBENOBUFS\fR\fR
.ad
.RS 11n
Insufficient resources were available in the system to perform the operation.
.RE

.sp
.ne 2
.na
\fB\fBENOMEM\fR\fR
.ad
.RS 11n
Insufficient memory was available to fulfill the request.
.RE

.sp
.ne 2
.na
\fB\fBENOSR\fR\fR
.ad
.RS 11n
There were insufficient STREAMS resources available for the operation to
complete.
.RE

.SH ATTRIBUTES
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Standard
_
MT-Level	MT-Safe
.TE

.SH SEE ALSO
.LP
.BR poll (2),
.BR select (3C),
.BR socket.h (3HEAD),
.BR recv (3XNET),
.BR recvfrom (3XNET),
.BR send (3XNET),
.BR sendmsg (3XNET),
.BR sendto (3XNET),
.BR setsockopt (3XNET),
.BR shutdown (3XNET),
.BR socket (3XNET),
.BR tcp (4P),
.BR attributes (7),
.BR standards (7)
